Twelve people, including women and children, are being treated for possible exposure to chemical weapons agents in Mosul, where Islamic State is fighting off an offensive by U.S.-backed Iraqi forces, the United Nations said on Saturday.

The U.N.'s World Health Organization has activated with partners and local health authorities "an emergency response plan to safely treat men, women and children who may be exposed to the highly toxic chemical," the agency said in a statement.

It said all 12 patients had been received since March 1 for treatment which they are undergoing in Erbil, the capital of Iraq's Kurdish region, east of Mosul.

Four of them are showing "severe signs associated with exposure to a blister agent". The patients were exposed to the chemical agents in the eastern side of Mosul.

The International Committee of the Red Cross (ICRC) said on Friday that five children and two women were receiving treatment for exposure to chemical agents.

The ICRC statement did not say which side used the chemical agents that caused blisters, redness in the eyes, irritation, vomiting and coughing.

Iraqi forces captured the eastern side of Mosul in January after 100 days of fighting and launched their attack on the districts that lie west of the Tigris river on Feb. 19. The eastern side remains within reach of the militants' rockets and mortar shells.

Defeating Islamic State in Mosul would crush the Iraqi wing of the caliphate declared by the group's leader, Abu Bakr al-Baghdadi, in 2014, over parts of Iraq and Syria.

The U.N. Humanitarian Coordinator in Iraq, Lise Grande, called for an investigation.

"This is horrible. If the alleged use of chemical weapons is confirmed, this is a serious violation of international humanitarian law and a war crime, regardless of who the targets or the victims of the attacks are," she said in a statement.